1. Field of the Invention
The present invention relates to a signal output circuit to be incorporated into a MOS semiconductor integrated circuit (IC) designed to drive bus lines. More particularly, it relates to a signal output circuit made in the form of a Bi-MOS IC comprising bipolar transistors and MOS transistors.
2. Description of the Related Art
An Bi-MOS signal output circuit which outputs signal at a TTL (Transistor-Transistor Logic) level has two bipolar transistors at its output. These bipolar transistors are controlled by the signal output by a control circuit comprising MOS transistors.
FIG. 1 shows a conventional signal output circuit of this type, which comprises an N-channel MOS transistor 11, a Schottky-junction NPN-type bipolar transistor 12, a resistor 13, and an NPN-type bipolar transistor 14. While the input signal IN is at a "H" level, the N-channel MOS transistor 11 remains on. In this condition a base current flows from a power-supply potential VCC through the resistor 13 to the NPN-type bipolar transistor 12 and the bipolar transistor 12 is turned on. As a result of this, the node of the bipolar transistors 12 and 14, i.e., the output terminal, is discharged to the ground potential GND, whereby the output signal OUT is set at a "L" level.
On the other hand, while the input signal IN is at the "L" level, the MOS transistor 11 remains off. In this condition, a base current flows from the power-supply potential VCC through the resistor 13 to the NPN-type bipolar transistor 14, and the transistor 14 is therefore turned on. Hence, the node of the transistor 12 and 14 is charged with the power-supply potential VCC, whereby the output signal OUT is set at the "H" level.
As is shown in FIG. 1, the conventional signal output circuit further comprises a pull-down circuit 15 which is connected between the ground potential GND and the node of the MOS transistor 11 and the bipolar transistor 12. The pull-down circuit 15 discharges the base of the bipolar transistor 12 to the ground potential GND when the MOS transistor 11 is cut off, thus setting the base of the bipolar transistor 12 into a floating state. Its base thus discharged, the bipolar transistor 12 is quickly turned off.
To increase the output sink current required to set the output signal OUT at the "L" level, it suffices to increase the base current of the bipolar transistor 12. To increase this base current, it suffices to use, as resistor 13, a resistor having a low resistance. Thus, the power of this signal output circuit consumes increases in proportion to the output current.
The lead electrodes and the like of an IC package incorporating the signal output circuit (FIG. 1) have inductance components. When the signal output circuit drives a load having the inductance components and also capacitance components, the waveform of the output signal OUT of the signal output circuit will likely to contain a ringing component. Nevertheless, the bipolar transistor 12 can absorb the ringing component sufficiently since it exhibits non-linear characteristic and has a high resistance when the output signal OUT is at about 0 V. In other words, the signal output circuit (FIG. 1) outputs a signal containing, if any, an extremely small ringing component.
FIG. 2 illustrates another conventional signal output circuit which comprises an NPN-type bipolar transistor 21, an N-channel MOS transistor 22, an NPN-type bipolar transistor 23, and an N-channel MOS transistor 24. While input signals IN, IN are at the "H" level and the "L" level, respectively, the NPN-type bipolar transistor 21 remains on, whereby the node of the bipolar transistors 21 and 23 is charged by the power-supply potential VCC. As a result, the output signal OUT is set at the "H" level. Conversely, while the input signal IN is at the "H" level, the N-channel MOS transistor 22 is on, whereby a base current flows from the node of the transistors 21 and 23 to the bipolar transistor 23. Hence, the bipolar transistor 23 is turned on, and the node of the transistors 21 and 23 is discharged to the ground potential GND. Meanwhile, upon receipt of the input signal IN at the "H" level, the MOS transistor 24 is turned on, and the node is also discharged to the ground potential GND also via this MOS transistor 24. Therefore, the node of the bipolar transistors 21 and 23 is discharged to the "L" level through two transistors, i.e., the bipolar transistor 23 and the MOS transistor 24.
The signal output circuit shown in FIG. 2 further comprises a pull-down circuit 25 which performs the same function as the pull-down circuit 15 incorporated i the signal output circuit illustrated in FIG. 1.
In the conventional circuit shown in FIG. 2, in order to set the output signal OUT at the "L" level, a current is accumulated in two current paths formed of the bipolar transistor 23 and the MOS transistor 24, respectively. Unlike in the signal output circuit shown in FIG. 1, no currents always flow between the power-supply potential VCC and the ground potential GND. Therefore, the circuit of FIG. 2 consumes but power as little as is consumed by a CMOS logic IC. In addition, the sink current required to set the output signal OUT at the "L" level can be increased, merely by using a large element for the bipolar transistor 23, which therefore has a low on-resistance. Hence, the circuit does not consume as much power as does the signal output circuit illustrated in FIG. 1. However, the circuit is disadvantageous in that the bipolar transistor 23 cannot absorb the ringing component of the output signal when the circuit drives a load containing inductance components and capacitance components. This is because the on-resistance of the bipolar transistor 23 is low when the output signal OUT is at about 0 V. In other words, the output signal OUT is very likely to have a waveform containing a ringing component.
As has been pointed out, either conventional signal output circuit described above can accomplish both things required of this kind of circuit, i.e., an decrease in power consumption and a reduction in the ringing component in the output signal.